Lung cancer is the leading cause of cancer related mortality in both men and women and remains a major health issue. More than 160,000 individuals will die from lung cancer in the coming year, more than breast, prostate and colon cancer combined. The majority of lung cancer cases is attributable to tobacco smoking and in some cases other environmental risk factors. Although the relative risk of developing lung cancer declines dramatically in smokers who quit, former smokers remain at risk for the disease. Several recent studies show that greater than 50% of newly diagnosed lung cancers occur in former smokers. Of the tumors detected in former smokers, nearly 50% occurred in patients who had quit for more than five years. It is estimated that there are approximately equal numbers of smokers and former smokers in the United States. Since smoking cessation is a major public health initiative, former smokers will increasingly account for a higher percentage of lung cancer cases. Thus, two high-risk population groups exist for lung cancer and improved disease management can be beneficial to both current and former smokers. Additionally the prognosis for lung cancer patients is very poor, as reflected by an overall, 5-year survival rate of only 14%. The poor prognosis for lung cancer patients is due, in part, to the historical lack of effective early detection measures. TUMOR SUPPRESSOR GENES (TSG) ON CHROMOSOME 9P: Chromosome 9p deletions and alterations occur early and often in lung cancer. The p16/CDKN2 locus, located on 9p, is suspected to be the major tumor suppressor gene inactivated in this tumor type. However, we have previously identified a region of homozygous deletion on the short arm of chromosome 9p at the microsatellite marker D9S126. The minimal region of deletion detected was approximately 600 Kb, is distinct from the p16/CDKN2 tumor suppressor gene locus, and lies approximately 2 cM proximal. We proposed that the region harbors a TSG important in lung tumorigenesis. We furthered our analysis by using immunohistochemistry to analyze the expression of p16 in adenocarcinomas and squamous cell carcinomas of the lung and subsets of these tumors were analyzed for loss of heterozygosity (LOH) with microsatellite markers spanning the short arm of chromosome 9. The immunohistochemistry revealed a significant difference in the percent of tumors positive for p16 with the adenocarcinomas having a higher percentage of positive staining than the squamous cell carcinomas. Loss of heterozygosity analysis demonstrated that the pattern of loss was similar between p16 positive and negative squamous cell carcinomas. However, there were moderately significant differences in the LOH analysis between p16 positive adenocarcinomas and squamous cell carcinomas at some of the markers. More recently, 30 non-small cell lung cancer and 12 small cell lung cancer cell lines were screened with 55 markers to identify new regions of homozygous deletion on chromosome 9p. Three novel non-contiguous homozygously deleted regions were detected and ranged in size from 840 Kb to 7.4 Mb. One of these regions included the marker D9S126 and one gene identified in the deletion was TUSC1. Multiplex PCR and Southern blot confirmed the homozygous deletion of TUSC1. Northern blot analysis of TUSC1 demonstrated two transcripts of approximately 2 and 1.5 kb that are likely generated by alternative polyadenylation signals. Both transcripts are expressed in several human tissues and share an open reading frame encoding a peptide of 209 amino acids. Analyzing lung cancer cell lines for RNA and protein expression demonstrated down regulation of TUSC1 in several cell lines suggesting TUSC1 may play a role in tumorigenesis.